An optical isolator element includes a, Faraday rotator, and a first polarizer and a second polarizer sandwiching therebetween the Faraday rotator. The optical isolator element transmits light in a first direction (hereinafter referred to as light in a forward direction) as it is, and blockings light in a second direction opposite to the first direction (hereinafter referred to as light in a backward direction). There also exists, as the optical isolator element, a multi-layered type optical isolator element that is constituted by sequentially arranging a first polarizer, a first Faraday rotator, a second polarizer, a second Faraday rotator and a third polarizer, and has the same function.
Commonly, an absorption type polarizer is used in such polarizer that constitutes an optical isolator element. The absorption type polarizer is obtained by forming a crystal of a halide of copper, silver or cadmium in glass, uniaxially drawing the crystal by heating, and reducing the drawn crystal in a hydrogen atmosphere thereby precipitating a needle-shaped metal particle in the glass (see, for example, JP-A-08-50205).
However, in the optical isolator element using such polarizer, it is impossible to increase the concentration of a metal halide in the glass to a given concentration or higher. Therefore, in order to obtain requisite extinction-ratio characteristics, it is necessary to increase the thickness of a layer in which metal particles are distributed. Usually, a metal particle layer is distributed from a surface of a polarizer to the depth of 50 μm. The amount of metal particles is ensured by increasing the distribution depth, and thus desired extinction-ratio characteristics are obtained. Therefore, there is a problem that it is difficult for this type of a polarizer to decrease the thickness.
When the polarizer has a large thickness, there is a problem that an optical isolator element cannot be downsized. In case of light beam wherein a spot size of incident light is gradually increased, when the pot size increases more than that of the optical isolator element, there arises vignetting in wherein ambient light in the spot is blocked, resulting in optical loss. Therefore, it was required to sufficiently increase the size of the optical isolator element more than the spot size. Accordingly, there was a problem that it is difficult to miniaturize the optical isolator element.
There also exists, as the absorption type polarizer, a absorption type polarizer having a polarization function obtained by embedding metal particles on a glass surface using a physical vapor deposition method such as sputtering, and drawing the metal particle-embedded glass thereby orienting the glass (for example, JP-A-09-178939).
When an optical isolator element is produced using this type of a polarizer, the distribution depth of metal particles from the glass surface can be decreased and thus the thickness of the polarizer can be decrease. In compensation therefor, in order to obtain desired extinction-ratio characteristics, it is necessary to endure the amount of metal particles by increasing the metal particle density of the glass surface.
However, when the density of metal particles on glass surface is increased, a refractive index on the glass surface increases and light reflections may be sometimes caused by a difference in the refractive index. In this case, for example, as shown in FIG. 9, it is assumed that light 91 in a backward direction is incident on an optical isolator element 60 from a first polarizer 62 side. While light 91 in a backward direction is blocked by a second polarizer 63, reflected light 92 reflected partially at an interface 63a having a high metal particle density of the second polarizer 63 may sometimes reflect again at the interface 62a having a high metal particle density of first polarizer 62, and emit outside from the second polarizer 63 side as outgoing light 93. Since light in a backward direction is partially transmitted through the optical isolator element 60 in this way, there arises a problem that isolation characteristics deteriorate.